1. Field of the Invention
The present invention relates to a method of making sodium hydroxide. In another aspect, the present invention relates to a process for the production of sodium hydroxide from trona ore. In still another aspect, this invention relates to the control of calcium carbonate crystal size to facilitate the separation of the solid calcium carbonate from the desired liquid sodium hydroxide product, by utilization of sodium carbonate during slaking.
2. Description of the Related Art
Sodium hydroxide is a white, somewhat translucent crystalline solid, which is also known widely in the industry as "caustic soda". A major use of sodium hydroxide is to form sodium salts, thus neutralizing strong acids and solubilizing water-insoluble chemicals through the formation of the sodium salt. Sodium hydroxide is also useful in the precipitation of heavy metals as their hydroxides, and in the control of acidity of aqueous solutions.
Because of its varied chemical activity, sodium hydroxide finds utility in a wide variety of processes. For example, sodium hydroxide is utilized in processes of making tribasic sodium phosphate, sodium chlorite, sodium chloroacetate, sodium cyanide and sodium formate. Sodium hydroxide is also utilized in the refining of kraft process pulp to higher content alpha cellulose, petroleum refining, manufacture of detergents, manufacture of soaps, textile processing, and metal processing. In addition, sodium hydroxide also finds utility in refining vegetable oils, water and acid waste stream treatment, pH control, alkaline bottle washing formulations and in groundwood pulp bleaching.
Sodium hydroxide has been produced from soda ash since shortly after the development of the Leblanc synthetic soda ash process in the early 18th century. In this lime-soda process, dry, pure soda ash (sodium carbonate) was dissolved in water and mixed with milk of lime (hydrated lime in water) to form sodium hydroxide in solution and calcium carbonate solids. Separation of the solids from the liquor resulted in a 10 to 12 percent sodium hydroxide solution that could be used directly or concentrated to a desired concentration. However, this process utilizes very pure sodium carbonate, i.e., greater than 99.5% sodium carbonate, and is known for sodium hydroxide product that is low in metals, organic carbons and insoluble impurities. The principle disadvantage of this process is that it requires an extremely pure soda ash feed, which is economically prohibitive.
When the electrolytic process for caustic and chlorine was developed, this process could compete only with great difficulty and its use has gradually dwindled. The lime-soda process is no longer used to produce caustic soda for sale, but is still used by industry in processes such as kraft recovery and to a limited extent in the production of alumina, in which lime and soda ash are charged to barrite digesters. Under the digester operating conditions, the reaction is not complete, resulting in a loss of efficiency and other difficulties. Consequently, most alumina plants prefer to charge caustic soda directly to the digesters.
Since the development of the electrolytic process over 50years ago, the lime-soda process has slowly been displaced. In the electrolytic process, saturated sodium chloride brine is fed to an electrolytic cell where 10 to 12 percent sodium hydroxide, along with hydrogen gas are produced at the cathode, and chlorine is produced at the anode. The reaction may be expressed as: ##STR1##
The sodium hydroxide thus produced may be concentrated to the desired concentration, generally from 50 to 73 weight percent, by evaporation. During this evaporation process, most of the unreacted sodium chloride crystallizes, is separated from the caustic and is recycled back into the electrolytic cell feed system.
This process is economically viable if the chlorine by-product can be sold. However, in recent times the demand for chlorine has not maintained a balance with sodium hydroxide demand. Environmental concerns related to chlorine containing compounds impact the demand for chlorine as the use of chlorine containing compounds is curtailed or prohibited. Examples of such chlorine containing compounds include vinyl chloride, a number of highly chlorinated high volume pesticides, ethylene dichloride as lead scavengers in gasoline, fluorocarbon aerosol propellants, polychlorinated biphenyls, and chlorine and chlorine-containing chemicals used water and sewage treatment.
Accordingly, the need exists in the industry for an economical process for the production of sodium hydroxide, without chlorine coproduction.